The present invention is in a method for separating the residue obtained in the distillation of chlorosilanes from the other chlorosilanes, processing the residues to obtain a very complete hydrolysis of the residues and obtaining reusable hydrogen chloride.
The residues to be processed according to the invention occur in the production of chlorosilanes. Chlorosilanes are prepared by the reaction of chlorine or hydrogen chloride with raw silicon. Industrially, raw types of silicon having silicon contents of 85% and more, are usually used for this purpose. Other components of the raw silicon are mainly iron and aluminum. In the chlorination or hydrochlorination, the silicon and metals contained in the raw silicon are converted to chlorides and occur in granular form as byproducts. In addition to these metal chlorides, high-boiling components, such as titantium tetrachloride, hexachlorodisiloxane or pentachlorodisiloxane, also occur as byproducts in the production of chlorosilane.
The chlorosilanes are usually roughly separated from the solid residues by distillation, leaving as residue a suspension that requires separate processing. Reaction of this residue with water transforms the chlorosilanes contained in the residue to silicic acid and the metal chlorides to metal oxides or hydroxides, and releases hydrogen chloride in the form of hydrochloric acid. Disadvantages of this process include the loss of chlorosilane product and the formation of a hydrochloric acid containing solution of metal salts and silicic acid which must be disposed of.
DE-AS 21 61 641 discloses transferring the distillation residue into a mixer, distilling out the chlorosilanes to obtain a residue low in chlorosilane and treating it with steam. The distilled off hydrogen chloride is fed to an absorption process and the hydrolysis residue is removed dry. A disadvantage of that process is that by the addition of a small amount of steam, one obtains a highly concentrated hydrogen chloride, but an incompletely hydrolyzed residue which still contains a considerable residual chlorine content requiring further processing. If a large amount of steam is added, the residual chlorine content in the residue is low, but the distilled out hydrogen chloride contains excess water vapor. Since the formation of the azeotrope makes complete separation of hydrogen chloride and water impossible, the water fed to the following separation in the recovery of hydrogen chloride has to be let out as hydrochloric acid. This hydrochloric acid is no longer usable in the production process in question and constitutes an industrial product of lesser value, while the hydrogen chloride it contains can be used in an isolated form.
Another disadvantage of this process is that the drying and hydrolysis takes place in the same apparatus. Graded temperatures must be used to prevent aluminum chloride from issuing at the top of this apparatus. The drying is performed at low temperatures and the hydrolysis at high temperatures. Thus the process becomes laborious.
The problem therefore existed of finding a method for processing the distillation residues with the recovery of chlorosilanes and hydrogen chloride, capable of delivering a hydrolysis residue suitably low in chlorides so it may be dumped, while permitting a maximum recovery of hydrogen chloride. Further the recovered hydrogen chloride should be of such a purity that it can be directly reused, e.g., in the present process. Further production of additional hydrochloric acid should be prevented because its economical disposal is difficult.